In optical communication systems and in particular in long haul optical communications systems, dispersion effects, if uncorrected, cause significant bit error rates. Thus, a large variety of approaches have been developed to deal with such dispersion. (See Optical Fiber Telecommunications 111A, Chapters 6 and 7, Ed. by I. P. Kaminow and T. L. Koch, Academic Press, 1997 for a general description of dispersion effects in optical communication systems.) These approaches for correcting dispersion have been effective and as a result have allowed an increased rate of information transmission as well as an increased distance between points at which the signal is reshaped. However, as transmission pulse rates become faster and/or as distances between reshaping become greater, new effects resulting in dispersion become significant. In particular, at data repetition rates above about 40 Gb/s and/or signal reshaping spaces greater than about 500 km polarization mode dispersion begins to present a concern.
Generally light launched for long haul communications on an optical medium such as a laser light from a distributed feedback laser, contains essentially only one polarization mode. Nevertheless coupling in the fiber soon produces two polarization modes with the injected light power divided between these two modes. Since the two modes do not travel at the same rate through the optical medium, the information contained in one polarization mode becomes spread in time relative to the other mode as it traverses the optical medium. Thus polarization mode dispersion adds to other undesirable dispersive effects.
For most optical media the difference in traversal rate between the two polarization modes, commonly denominated TE and TM, is relatively small—on the order of a few picoseconds. However, as previously discussed, at repetition rates approaching about 40 Gb/s or reshaping distances approaching about 500 km, even the relatively small difference between the traversal rates of the two polarization modes becomes meaningful. Thus, an increased interest in correcting polarization mode dispersion has been generated.
Before compensating for polarization mode dispersion, the polarization state of the incoming optical signal is desirably brought to the state of polarization that is advantageous for correction by the device being employed. Numerous approaches have been developed in optics to change the state of polarization in an incoming optical wave into a second desired state of polarization. For example, the sequence of a first quarter wave plate, a half wave plate, and a second quarter wave plate intercepting the light is employable to produce the transition from the input polarization state to a second desired output state. (See Heismann, Journal of Lightwave Technology, 12 (4), 696 (1994), which is hereby incorporated by reference in its entirety, for a description of this optical plate arrangement.) Each plate is rotated sufficiently around an axis through the center of and perpendicular to its major surface to achieve the desired conversion. For example, as shown schematically in FIG. 1, the three plates as indicated are rotated to an appropriate degree to produce the desired conversion. (The necessary degree of rotation for a given input state and desired output state is obtained as described in Heismann, supra. By convention, the rotation angle of the first quarter wave plate is denominated α/2, the rotation of a half wave plate is denominated γ/2, and the rotation of the third wave plate denominated β/2.) However, the mechanical rotation of wave plates in response to an incoming signal having rapidly changing polarization states is not a practical approach to providing a desired output polarization state for dispersion correction.
A variety of devices that are the equivalent to the previously described three wave plate device but whose effect on polarization state is controlled by manipulation of an electrical signal have been proposed. For example, as discussed by Heismann, supra, a device equivalent to the three wave plate configuration is producible in a lithium niobate wafer. This device has the attribute of relatively fast conversion from one polarization state to another—speeds reported to be on the order of 4900 rad/s accompanying reset free polarization. (Reset free involves the operation of a device to convert a communication signal with varying incoming polarization states to desired output polarization states without abruptly changing the device control signal even during extremes in operation, such as when the control signal is operated at the physical limits of the device. Resets are typically implemented by introducing additional redundant components into the device that are only switched on during times when the reset occurs but are otherwise idle. During the reset, the incoming and outgoing polarization states are necessarily static. This pause is problematic in situations when the controller must track polarization changes occurring at fast speed.) Although such lithium niobate devices are indeed reset free and have conversion speeds significantly faster than mechanical wave plate configurations, they, nevertheless, rely on fabrication by techniques less adapted for mass production than that typically used in integrated circuit manufacture.
In an attempt to reduce cost and increase speed, polarization controllers for use in polarization mode dispersion compensation have been proposed to be fabricated as a silica based planar lightguide circuit. (See T. Saida et. al. IEEE Photonics Technology Letters, 14(4), 507 (2002)). Fabrication of devices such as tunable couplers, phase shifters, and polarization beam splitters are well-known as discussed in Optical Fiber Telecommunications IIIB, Ed. By Kaminow and Koch, Chapter 8 by Li and Henry. The phase shifters and tunable couplers produced using a substrate having silicon and silicon dioxide regions are controlled by electrodes configured to produce localized heating in appropriate device regions. The degree that the phase is shifted or the extent of coupling between light in two individual waveguides depends on the degree of heating. Clearly, material properties limit the extent of heating that is acceptable. Since fabrication of individual components forming the polarization controller is well known and fabrication in a silicon/silica based medium is relatively inexpensive, such device has the potential for being significantly easier to fabricate than a device based on lithium niobate. Additionally, the speed of such device, as demonstrated by Saida, et. al, is substantially faster than manual manipulation of wave plates, although improvement would be desirable for optical communication. Nevertheless, these silicon/silica based devices are not reset free, contain redundant components to convert polarization states and suffer polarization delays associated with the reset. To mitigate such polarization delays, Saida, et. al. supra has suggested use of a dithering approach for controlling the device. In such an approach the control signal is randomly oscillated until a chosen parameter such as output signal strength is improved to a desired level. However, dithering relies on an interactive feedback approach from an initial control system setting. Such approach is less desirable in demanding operations since reaching the desired control signal is slowed by the iterative process.
There is therefore a need for a device that 1) is reset free in the sense that it is possible to track changes in the incoming and outgoing polarization states with corresponding changes in the control signal without exceeding the physical range limits of the device, 2) is based on a material system such as silicon/silica which provides conventional fabrication, and 3) compensates polarization mode dispersion as part of, for example, a polarization mode dispersion compensator. In an even more desirable configuration, the device would be not only reset free, but also fully deterministic. (Fully deterministic in this context means that the control parameter settings for the device are not dependent on dithering so that the desired conversion from input to output state of a wave plate is achievable during operation to an accuracy of 5 percent or better without an iterative process. For example, the desirable accuracy is achieved by setting the control signal based on a calculation from analytical equations using information about the desired input and output polarization state.)